Nowadays, integrated circuits (ICs) may comprise a plethora of sensors, such as gas sensors, relative humidity (RH) sensors, specific analyte detection sensors, and so on. Such sensors may be included in the IC design for a number of reasons.
For instance, a CO2 sensor may be included in an IC to detect a change in the ambient conditions of a product tagged with the chip such that product quality control can be achieved by monitoring the sensor readings of the chip. This can for instance be used to accurately predict the remaining shelf life of the product, e.g. perishable food stuff, a pharmaceutical or biomedical product, or logistics-based applications. The sensor may for instance be adapted to determine changes in the CO2 content of the ambient atmosphere. Alternatively, the sensor may be used to detect changes in the CO2 composition of larger environments such as buildings, e.g. in heating ventilation and air conditioning (HVAC) applications, or may be used in medical application domains, e.g. in breathing apparatuses.
It is particularly relevant to mass market applications such as RF tags for product monitoring that the gas sensor functionality can be added to the IC with limited additional cost, as there is a large price pressure on such ICs; i.e. they have to be produced cheaply in order to be commercially attractive. This is not easily achieved, especially when there is a requirement for the sensor to have high, e.g. parts per million (ppm), CO2 sensitivity, such as in HVAC applications where CO2 levels may be below 1,000 ppm.
David J. Heldebrandt et al. in Energy Procedia 1 (2009), pages 1187-1195 disclose a new class of CO2 absorbing materials, referred to as CO2-binding organic liquids (CO2-BOLs) that are neat (solvent-free) liquid mixtures of organic alcohols and organic amidine or guanidine bases, which undergo the following reversible reaction in the presence of CO2:

The CO2 may be released by purging the CO2-BOLS e.g. with N2. In the above reaction scheme, DBU (diazabicyclo[5,4,0]undec-7-ene) is shown as the amidinium precursor, although Heldebrandt et al. disclose that a large variety of amidines and guanidines exhibit similar sensitivity to CO2. Similarly, several alkyl alcohols, e.g. hexanol, may be used to form the alkylcarbonate anion. The CO2 uptake was determined using conductivity measurements of the CO2-BOL dissolved in acetonitrile, as the level of CO2 uptake is (linearly) correlated to the conductivity of the solution. The authors recommend that the absorption capacity of a CO2-BOLS can be increased by choosing a base and alcohol of low molecular weight, e.g. 1,1,3,3-tetramethylguanidine (TMG) and methanol. However, a challenge remains as to how to integrate a CO2-BOL as a sensor material on an integrated circuit.